<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>新增长度单位</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }
    .box1{
      width: 200px;
      height: 200px;
      background-color: skyblue;
      padding: 5px;
      border: 5px black solid;
      margin-bottom: 20px;
      /* 正常盒子模型:内容区 = width */
    }
    .box2{
      width: 200px;
      height: 200px;
      padding: 5px;
      border: 5px black solid;
      background-color: green;
      box-sizing: border-box;
      /* border-box：width = padding + border + 内容宽度 */
    }
  </style>
</head>
<body>
  <div class="box1"></div>
  <div class="box2"></div>
  
</body>
</html>